A question of obvious practical importance is how modifiable is the natural human wearout lifespan (the expected lifespan if senescene were the only cause of death)--i.e., how easily can the lifespan be either extended or reduced by repairing, replacing, or damaging the components that comprise the system. The extensibility--the ease of extending the wearout lifespan--of a series system is an increasing function of the variation of wearout lifetimes of its components, whereas the reducibility--the ease of reducing the lifespan--is a decreasing function of the lifetime variation. Thus, the wearout lifespan of a system cannot in general be both easy to extend and easy to reduce. The lifetime variation of biological systems, and therefore which of these opposite possibilities will obtain for humans, is a product of natural selection. I have conclusively demonstrated the existence of two, probably central natural-selective forces on lifetime variation: synchronism of aging, which tends to decrease the variation, and antisynchronism of aging, which tends to increase it. Moreover, the net lifetime variation-and hence the resultant lifespan modifiability of the system--is the result of a balance between these opposing forces that is determined by the detailed structure of the system. Consequently, if knowledge were available regarding the net effect of system structure on evolved lifetime variation in complex general systems, the resulting inferences regarding lifespan modifiability could be applied to isomorphic real systems. The specific aim of this proposal is to conduct such an analysis. The key to the research is a fully developed simulation model of the evolution of wearout in complex systems that allows synchronism/antisynchronism to be analyzed both experimentally and mathematically. Critical experiments and analyses are proposed for several abstract systems of biological interest, and means are proposed for applying the findings to real systems, especially humans. One application of the research would be in deciding which approach would be more likely to succeed: public health practices to prevent reduction of the lifespan or gerontological intervention efforts to extend it.